The present invention relates to polyamic acids which are used as intermediate products for the preparation of electrically insulating high-temperature resistant layers of polyimides. The polyimide layers serve, in particular, as protective layers and insulating intermediate layers for electronic components and circuits on semiconductors or metals.
The use of polyimides for high-temperature resistant layers on and in semiconductor devices is known, for example, from German Pat. No. 1,764,977. The polyimides are prepared by polycondensation of aromatic diamines, for example, 4,4'-diamino-diphenyl ether, with aromatic tetracarboxylic acid anhydrides, for example, pyromellitic dianhydride, to give polyamic acids. The polyamic acids are heated, thereby splitting off water and forming polyimide. These polyimides, if their molecular weights are high enough, have the required temperature resistance and the desired electrical properties. The solutions of the corresponding high-molecular weight polyamic acids, have a high viscosity, already at relatively low concentrations. These solutions of high-molecular weight polyamic acids are therefore processed with difficulty. The high-molecular weight polyimides as such are usually relatively brittle and can only insufficiently planarize, i.e., embed in a layer having a plane surface, the conductive paths covered by them.
A reduced viscosity at an unchanged molecular weight is obtained, when pyromellitic acid is entirely or partly replaced by benzophenone-3,3',4,4'-tetracarboxylic acid. Polyamic acids of that type and polyimides prepared therefrom are described in German Pat. No. 1,520,767. In the publication "Pyralin, Polyimide Coatings for Electronics", Bulletin PC-1 by E. I. du Pont de Nemours and Co., Wilmington, U.S.A., the improved planarizing effect of these polyimides is pointed out. The corresponding polyamide acids, however, tend at relatively low temperatures to enter into cross-linking reactions to form insoluble products. The processing range of these intermediate products is therefore limited. This disadvantage is more pronounced as the proportion of benzophenone-tetracarboxylic acid units increases. Therefore, the reduction in viscosity which can practically be achieved is limited. In addition, the thermal stability of polyimides based on pyromellitic acid derivatives is generally higher than the thermal stability of polyimides prepared from benzophenonetetracarboxylic acid derivatives.
The diamine components of the polyamic acids have also been varied, but particular changes of properties in connection therewith have not been described. For example, in U.S. Pat. No. 3,179,614, phenylene diamines, naphthylene diamines, xylylene diamines, substituted benzidines, diaminopyridines and diamino-diphenyl ethers, -diphenyl sulfides -diphenyl sulfones, -diphenyl methanes and -benzophenones are disclosed as diamine components. In addition to the 4,4'-isomers of the binuclear compounds, this patent also discloses 3,3'-diamino-diphenyl sulfone and 3,4'-diamino-biphenyl. Examples of the application of the binuclear compounds are only given for the 4,4'-isomers.